Add release notes for 3.1
README: add primus, safer gpasswd command
Updated RELEASE_NOTES_3_1

Be more descriptive about primus.

Changed date for real release date.
Merge branch 'release-3.1' of github.com:Bumblebee-Project/Bumblebee into release-3.1
Removed trailing spaces in release notes, typo fix
Merge branch 'release-3.1'